perf(web): preserve completed code-line DOM while streaming

[juliusmarminge]

Streaming code replaces the entire highlighted HTML on each chunk, making the browser rebuild unchanged token spans. Render completed lines through memoized components and preserve their DOM identity as new lines arrive. This carries Legend's stable document content approach into the existing web code renderer.

The incremental highlighter now returns a HAST document. Existing HAST serializers preserve the same markup, escaping and styles. Previously streamed blocks retain their line renderer through completion so selection survives; ordinary cached history keeps the existing HTML path. Positional keys are stable for appended lines. The two declared HAST packages were already transitive dependencies.

Five alternating production-browser rounds at 1440 × 1000, 10 warm-up updates and 40 measured updates each. The fixture starts with 200 TypeScript lines and appends one line per update.

Synchronous client update	Base, including incremental highlighting	This layer
Median of run medians	118.9 ms	103.6 ms
Median of run p95s	154.8 ms	127.8 ms

The median reduction is 12.9%; every paired run improved. Probe, raw samples and selection/scroll checks. This measures client rendering in a heavy synthetic fixture, not provider throughput. The remaining CSS/style cost still prevents smooth frame-rate streaming in this stress case. Full exploration and rejected experiments.

102 focused tests passed before integration. After rebasing onto current main, 94 focused Markdown/highlighting tests and 159 timeline tests pass, with web typecheck. Main removed some older tests between these runs. HTML parity checks cover colors, escaping, whitespace and blank lines. A regression test verifies that finishing a stream does not retokenize its completed prefix.

Browser checks in light and dark mode retained the selected word export, the token node, the pre element, the exact scroll offset of 7528 and its reading position through 40 updates and completion. The first prototype failed the completion check; this version retains the incremental renderer state to fix it. This affects chat code on web and Electron's shared renderer. Mobile, file/diff rendering, server/provider behavior and wire contracts are unchanged.

Before, matching completed code and reading position:

After:

Before streaming:

https://gh-file-drop-api-prod-mi5fy3sowv63ufte.pinglabs.workers.dev/f/75c2a0a429b71b1f/lines-before-realtime.webm

After streaming:

https://gh-file-drop-api-prod-mi5fy3sowv63ufte.pinglabs.workers.dev/f/61ae11060df0efda/lines-after-realtime.webm

Videos preserve Chrome screenshot timestamps, including gaps between paints. Recording is separate from timing. The selection/completion follow-up is recorded in the linked JSON evidence. The quoted timings were captured on the pinned original baseline before the stack's integration rebase; they are not presented as measurements of newer main. The integrated post-rebase browser checks also retained selection, token identity and exact scroll position through streaming and completion in light and dark mode. No probe or evidence is committed.

Model: GPT-6. Harness: Codex.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR changes the production chat renderer by introducing HAST-based, line-preserving rendering for streamed code and adds new runtime dependencies. It also includes a lint-suppression directive in the new renderer, so human review is warranted.

Notes:

• No code objects were reviewed. Approvability was decided on eligibility alone.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	13.6 KiB	—	15.1 KiB	✅
Codex	Thread snapshot wire	—	7.0 KiB	—	7.3 KiB	✅
Codex	Live turn WebSocket wire	—	6.5 KiB	—	7.8 KiB	✅
Codex	Live turn WebSocket decoded	—	57.0 KiB	—	66.4 KiB	✅
Codex	Live turn messages	—	8	—	21	✅
Claude	Total thread wire	—	13.6 KiB	—	15.1 KiB	✅
Claude	Thread snapshot wire	—	7.1 KiB	—	7.3 KiB	✅
Claude	Live turn WebSocket wire	—	6.5 KiB	—	7.8 KiB	✅
Claude	Live turn WebSocket decoded	—	57.8 KiB	—	66.4 KiB	✅
Claude	Live turn messages	—	9	—	21	✅

Baseline: unavailable · PR result: f378a49 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 113.9 KiB
• Claude decoded thread snapshot: 114.6 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

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• apps/web/src/components/ChatMarkdown.test.tsx

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📝 Walkthrough

Walkthrough

The web highlighting pipeline now uses HAST documents for incremental output. A new component renders highlighted lines while preserving streamed content. Completed streamed blocks retain line-preserving rendering, while non-streamed results continue using cached HTML.

Changes

Incremental highlighting

Layer / File(s)	Summary
HAST highlighting pipeline apps/web/src/lib/incrementalHighlighting.ts, apps/web/src/lib/incrementalHighlighting.test.ts, apps/web/package.json	The incremental API now returns HAST documents through codeToHast. Tests serialize results with toHtml. Required HAST utilities are added as production dependencies.
Line-preserving renderer apps/web/src/components/chat/HighlightedCodeLines.tsx, apps/web/src/components/chat/HighlightedCodeLines.test.tsx, apps/web/src/components/ChatMarkdown.tsx	HighlightedCodeLines renders HAST token content by line. ChatMarkdown selects HAST or HTML output and serializes non-streaming HAST results for caching.
Streaming state and validation apps/web/src/components/ChatMarkdown.tsx, apps/web/src/components/ChatMarkdown.test.tsx	Completed streamed blocks retain line preservation. Tests cover completed-line reuse, fallback recovery, and unchanged-fence behavior.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant ChatMarkdown
  participant createIncrementalHighlightedDocument
  participant HighlightedCodeLines
  ChatMarkdown->>createIncrementalHighlightedDocument: request incremental HAST document
  createIncrementalHighlightedDocument-->>ChatMarkdown: return highlighted HAST
  ChatMarkdown->>HighlightedCodeLines: render preserved lines
  HighlightedCodeLines-->>ChatMarkdown: produce pre/code markup

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Suggested reviewers: t3dotgg

Merge Risk: ⚪ Minimal · up to f378a

The streaming regression test now verifies initial highlighting, and no actionable merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 40.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 5 functions across 6 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the primary change: preserving completed code-line DOM during streaming.
Description check	✅ Passed	The description is detailed and covers the change, rationale, UI evidence, performance results, tests, scope, and validation. It does not use the template headings or include the checklist, but those …

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@apps/web/src/components/ChatMarkdown.test.tsx`:
- Line 127: Update the ChatMarkdown test to assert that the initial streaming
render invokes highlight with the incremental suffix before calling
highlight.mockClear(). Keep the existing completion assertion that no
highlighted code contains the completed declaration, and ensure the test
verifies both the initial suffix highlighting and unchanged completion behavior.

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📒 Files selected for processing (7)

• apps/web/package.json
• apps/web/src/components/ChatMarkdown.test.tsx
• apps/web/src/components/ChatMarkdown.tsx
• apps/web/src/components/chat/HighlightedCodeLines.test.tsx
• apps/web/src/components/chat/HighlightedCodeLines.tsx
• apps/web/src/lib/incrementalHighlighting.test.ts
• apps/web/src/lib/incrementalHighlighting.ts

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[juliusmarminge]

Direct main-versus-stack comparison is now complete: the four-PR stack reduces synchronous update time by 32.0% across the fixed streaming mix (960 measured updates per build, six counterbalanced rounds). Prose median: 18.575 → 14.700 ms; growing-code median: 174.425 → 114.775 ms. Both workloads improve in all six paired rounds.

#11169 is rebased above this PR. Its clear incremental benefit is cached navigation: 252.475 → 178.500 ms, with sampled blank frames in 48/48 switches before and 0/48 after. Its incremental streaming result is mixed across rounds.

Full comparison · Samples and scripts